Mar. 24, 2014

Oxygen gives us life, but have animals always needed it—in such abundance—to survive? To investigate what kind of environmental conditions were required for the transition from single-celled organisms to animals, researchers have turned to a creature known for its tub-scrubbing powers—the sea sponge.

“By the time animals appeared, over 80 percent of earth's history had elapsed,” says Daniel Mills, a graduate student who studies sea sponges at the Nordic Center for Earth Evolution in Denmark. “So there's this long delay between the origin of life and the origin of animals.”

While there are several hypotheses to explain the gap, a popular one suggests that for billions of years, oxygen levels were simply too low to support animal life. It wasn’t until approximately 800-635 million years ago, towards the end of the Proterozoic Eon, that there was a rise in the level of atmospheric oxygen.

For the first time, scientists reporting in PNAS have experimentally tested this hypothesis—and they used a modern sea sponge, commonly called the “breadcrumb sponge” (Halichondria panicea), to do it. Today’s sponges are physiologically very similar to the earliest animals, and thus are a good proxy for estimating oxygen requirements in the past.

The researchers collected the sponges from a Danish fjord and sealed them in aquariums filled with seawater in which they could measure and control the amount of dissolved oxygen available to the organisms. The team was surprised to find that they could lower the oxygen in the water all the way down to a range of 0.5 to 4 percent of what sponges typically breathe, without obvious harm to the animal. “It was still breathing, it was still eating, and growing, while under these low oxygen levels,” says Mills, one of the study's lead authors. Furthermore, because oxygen in the aquariums wouldn’t reliably stay below 0.5 percent, the bottom limit may be even lower.

That modern sponges can, for the most part, be perfectly happy with very little oxygen indicates that their ancient sponge counterparts—which arrived on the scene some 700-800 million years ago—could have been fine in an oxygen-poor environment, too. The findings suggest that oxygen wasn’t the limiting factor preventing the formation of the most basic of animal life. So what was?

One possibility is that a lot of time goes into building a multicellular animal, even a simple one like a sponge. “That's a big upgrade in complexity from a solitary microbe,” says Mills. “It could just be that it's a difficult evolutionary transition to make from a genetics standpoint, that it requires a lot of biological and genetic innovation,” plus some necessary ecological conditions, he adds.

So far, Mills has only tested the effects of low oxygen on adult sponges. He now hopes to investigate how oxygen affects adult sponge reproduction and how it might impact offspring. If sponges of all ages can tolerate less oxygen, then that would be even stronger evidence that oxygen didn’t trigger the appearance of animals—a profound conclusion to draw from a modest sea creature known for its use in the shower.